Don't depend on converters/libiconv if base system is built WITH_ICONV.
And fix build with a few popular ports.
To reduce the number of false positives try
$ ln -s /usr/lib/libc.so /usr/lib/libiconv.so
$ ln -s /usr/lib/libc.a /usr/lib/libiconv.a
as some ports explicitly add -liconv to LDFLAGS.
How-To-Repeat: $ cd www/firefox; make install
$ ldd /usr/local/lib/firefox/libxul.so | fgrep iconv

A commit references this bug:
Author: gerald
Date: Mon Dec 5 01:04:08 UTC 2016
New revision: 427827
URL: https://svnweb.freebsd.org/changeset/ports/427827
Log:
Remove gcc/files/patch-libcpp which has not been present on lang/gcc49
and lang/gcc48, but is something we have in lang/gcc47 and that lang/gcc
carried over from the days it was about GCC 4.7 (so surviving both the
transitions to GCC 4.8 and recently GCC 4.9).
The underlying issue was addressed upstream 2014-10-24 with r216679,
and in FreeBSD head 2013-09-06 by theraven@ who fixed fixed our
iconv.h to not include stdbool.h.
PR: 161417
Changes:
head/lang/gcc/files/patch-libcpp